1. Field of the Invention
The present invention relates to a micromechanical system and more particularly to a micromechanical system fabrication method using (111) single crystalline silicon.
2. Description of Related Art
A micromechanical system, also called micromachined system, is implemented by patterning and integrating particular portions of the system on a silicon substrate in a precise shape in micrometer scale using silicon fabrication processes. This is based upon semiconductor device fabrication techniques such as deposition of a thin film, etching, photolithography, and impurity diffusion and doping.
Micromechanical systems include the silicon accelerometers for sensing the acceleration of a moving body and the gyroscope for sensing the angular velocity of a rotating body.
Such micromechanical systems comprise of a moving parts and a stationary parts, which, when viewed on the cross section of the silicon substrate, are fabricated separated from the silicon substrate.
Conventionally the bulk micromachining method, in which a (100) single crystalline silicon or (110) single crystalline silicon is etched in an aqueous alkaline solution so as to fabricate a microstructure, and the surface micromachining method, in which polycrystalline silicon deposited on a silicon substrate is released at specified locations by a sacrificial layer etching technique, are used so as to fabricate a micromechanical system.
On the other hand, the present invention provides a micromachining method using (111) single crystalline silicon. In the present invention, micromachining method is also referred to as micromechanical system fabrication method.
FIG. 1 illustrates various planes of single crystalline silicon. FIG. 1 shows the (100), (110), and (111) planes in the single crystalline silicon, which has a cubic lattice structure.
Micromachining methods using the (100) single crystalline silicon or the (110) single crystalline silicon as the silicon substrate are conventionally well known. To use the (100) single crystalline silicon as the silicon substrate means to use a single crystalline silicon that is oriented in the direction of the (100) plane. This is implemented by using a silicon wafer that is cut from an ingot in the (100) plane during a wafer manufacturing process.
While the conventional micromachining methods using the (100) single crystalline silicon or the (110) single crystalline silicon are already well known, nothing about a micromachining method using (111) single crystalline silicon is known until now.
This is because studies on the crystallographic characteristics and the characteristics in fabrication steps with respect to the (111) single crystalline silicon, reflecting particular essential factors of a micromechanical system, different from the semiconductor device fabrication method, have not been actively conducted.
Accordingly, the present invention is directed to a method using the (111) single crystalline silicon to fabricate micromechanical systems that substantially obviates one or more of the limitations and disadvantages of the related art.
An objective of the present invention is to provide a micromechanical system fabrication method using (111) single crystalline silicon, wherein the crystallographic characteristics and characteristics in fabrication steps with respect to the (111) single crystalline silicon are utilized in fabricating a micromechanical system, thereby improving the micromachining technology.
Additional features and advantages of the invention will be set forth in the following description, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, a micromechanical system fabrication method using (111) single crystalline silicon, comprising: fabrication step, in which the (111) single crystalline silicon is utilized as a silicon substrate; reactive ion etching step, in which microstructures which have to be separated from the silicon substrate are patterned; and selective release-etching step, in which the microstructures are separated from the silicon substrate in an aqueous alkaline solution.
The step of reactive ion etching allows a thickness of the microstructures themselves and spacing between the silicon substrate and the microstructures separated from the silicon substrate to be defined and adjusted.
In the step of selective release-etching, the etch is selectively performed with respect to {100} planes and {110} planes, thus preventing the microstructures from being damaged, and the microstructures are readily separated from the silicon substrate due to slow etching characteristics of {111} planes that are left after the selective release-etching step in an aqueous alkaline solution.
The step of reactive ion etching comprises: a first reactive ion etching step of defining the thickness of the microstructures themselves; and a second reactive ion etching step of defining the spacing between the microstructures and the silicon substrate.